The invention concerns inking devices for greasy-ink printing, for use notably in printing presses such as offset and direct typographical printing presses for greasy ink printing.
In these machines, ink is spread on gravure plates by inking cylinders covered with soft material, in general synthetic rubber. The inking cylinders are adjusted to have a minimum but sufficient contact with the plate or gravure cylinder, and it is generally accepted that the film of ink trapped in the contact zone is separated into two films at the downstream end in the direction of rotation.
To obtain an imprint of good quality, the thus-spread greasy ink must have a relatively high static viscosity, as well as a certain tackiness.
Spreading of the ink is facilitated by the thixotropical properties of the inks which provide an important reduction in their viscosity during lamination of the ink, and the modern tendency is directed towards gels having favorable rheological properties. Greasy inks at present in use and which enable good quality printing generally have a dynamic viscosity comprised between 50 and 100 poises. The inking device for these inks must thus be capable of laminating an ink having a viscosity in the given range.
Lamination of the ink in the inking device consists of providing on the inking roller a film of ink as uniform as possible and sufficiently thin not to clog the gravure. From calculations based on the thickness of ink which must be transferred onto the paper or other support to obtain a correct imprint, the optimum thickness of this film must be of the order of 12 microns.
To obtain a good quality imprint in a greasy ink press, it is thus necessary to:
laminate a greasy ink at the speed of operation of the printing press and with a sufficiently high viscosity (of the order of 50 to 100 poises); PA1 form by this lamination a film of ink having a thickness of about 12 microns on an inking cylinder coated with soft rubber; PA1 ensure that this film is as regular as possible.
and
There are already known inking devices which satisfy the condition of providing a film of uniform thickness, but cannot fulfil the conditions of the first two operations. One of these inking devices as disclosed in Chambon U.S. Pat. No. 3,283,712 includes a wiping cylinder of hard material applied with pressure against an inking cylinder of soft rubber rotating at the tangential speed V.sub.T of the plate cylinder. The wiping cylinder has a peripheral wiping speed V.sub.E and rotates in the same direction as the inking cylinder, i.e. in the zone of contact of the two cylinders they move in opposite directions. A mass of ink is lodged in the space above the zone of contact of the two cylinders, and is held in this space by a scraper touching the upper part of the wiping cylinder. The ink is thus laminated during passage between the two cylinders.
Experience has shown that such an inking device operates correctly for a ratio of wiping V.sub.E /V.sub.T between 0 and 0.5 and that above 0.5, there is a generation of difficult-to-absorb vibrations which produce visible streaks in the imprint. Also, in its range of satisfactory operation, this device involves important losses of ink due to the difficulties of adjustment between the scraper, the wiping cylinder and the lateral cheeks of the ink trough. Experience also shows that to obtain a film of ink having a thickness of 12 microns on the inking cylinder, the maximum permitted viscosity for the ink is 10 poises, which value is not compatible with greasy inks so tht use of such a device is limited to fluid inks.
A second known inking device employs even more fluid inks. This device includes a wiping cylinder of very small diameter, resembling a rod, for the purpose of facilitating lamination of thin films, which is slowly rotated in the opposite direction to the inking cylinder. Such a device has the advantage of being simple to provide but experience once again shows that only a low maximum viscosity of ink may be used; it is of the order of 3 poises. It is thus evident that this latter inking device is not suitable for printing with a greasy ink.
The present invention aims to remedy the drawbacks and insufficiencies of the known inking devices by providing a device of simple conception enabling lamination of a relatively high viscosity ink and which may be used in printing presses of the typographical and offset litho types to give an excellent imprint.